Download Development and Structure of the Excretory System The Nephron

Biology 224
Human Anatomy and Physiology II
Week 7; Lecture 1; Monday
Dr. Stuart S. Sumida
Development and Structure
of the Excretory System
The Nephron
DEVELOPMENT
AND
STRUCTURE OF
EXCRETORY
SYSTEM
EXCRETORY SYSTEM REVIEW
• Kidneys derived from INTERMEDIATE
MESODERM.
• Kidney starts out as a SEGMENTAL
STRUCTURE.
• Bladder, as part of embryonic gut tube: lining
derived from endoderm.
• Note, this is EXCRETION, NOT
“ELIMINATION.”
EARLY KIDNEY
DEVELOPMENT
• There is a segment of intermediate
mesoderm for every body segment.
• Earliest kidney appears in the cervical
region of the body! (About week 3.)
Called the PRONEPHROS.
• Develop very close to the gonads. (They
battle it out for the nearby ducts.)
THE PRONEPHROS
• The early anteriorly developed kidney parts.
• Has NO EXCRETORY FUNCTION.
• Functions to INDUCE DEVELOPMENT of
middle segments of intermediate meosderm
into the MESONEPHROS.
THE MESONEPHROS
• Some think it is the functioning embryonic
kidney. Some think is has no excretory
function.
• WE DO KNOW that the duct that attaches
to it (THE MESONEPHRIC DUCT) is very
important in INDUCING DEVELOPMENT
OF THE CAUDAL KIDNEY SEGMENTS
(the METANEPHROS).
Mesonephric Duct reaches all the way to end of gut
tube (cloaca).
We need to...
• Attach the ducts to the hindend
kidney (the metanephros).
• Split the bladder away from the gut
tube.
After the mesonephric duct attaches to cloaca, the
embryonic URETER grows from caudal to cranial to
attach to mass of metanephric kidney.
A septum, the URORECTAL SEPTUM, grows
between the more dorsal part of the gut tube and the
more ventral part that will become the bladder.
Note that attached to the bladder are right and left
ueters, the allantois (an extraembryonic membrane).
So, like the heart,
and the lungs, the
bladder is ventral
to your gut tube.
ASCENT OF THE KIDNEYS
• Kidneys don’t stay at caudal end of body.
• They “ascend” to a position just caudal to
diaphragm and liver.
• Right kidney is a bit lower due to mass of
liver.
• They work their way up by attaching to
successive segmental arteries.
They
“ascend” to
a position
just caudal
to
diaphragm
and liver.
Right kidney
is a bit lower
due to mass
of liver.
They work their way up by attaching to successive
segmental arteries.
BASIC
KIDNEY
STRUCTURE
Note: Large
vessels usually
ventral to ureter
exit.
KIDNEYS ARE
RETROPERITONEAL!!
(They do move a
reasonable amount when
you breathe. -- This is
why they can REALLY
hurt when they have
problems.
Retroperitoneal pain can
be extreme.)
KIDNEY IN
SECTION:
Outer CORTEX
Inner MEDULLA
DUCT SYSTEM OF KIDNEYS
• URETER runs from kidneys to urinary
bladder.
• Ureter ends in a dilated RENAL PELVIS.
• Renal pelvis branches into structures called
the MAJOR CALYCES (singular = calyx).
• Major calyces divided into MINOR
CALYCES.
• Minor calyces receive fluid outflow from
many microscopic collecting tubules.
URETER
runs from
kidneys to
urinary
bladder.
Ureter
ends in a
dilated
RENAL
PELVIS.
Renal
pelvis
branches
into
structures
called the
MAJOR
CALYCES
Major
calyces
divided into
MINOR
CALYCES.
Minor
calyces
receive fluid
outflow from
many
microscopic
collecting
tubules.
STRUCTURE OF THE BLADDER
• Sort of a bulging tetrahedron in shape.
• 4 ATTACHMENTS - one at each corner.
• One corner lies at top edge of pubic
symphysis (here, vestigal URACHUS holds
it down)
• Right and left URETERS dump in craniodorsally.
• URETHRA exits caudally (inferiorly).
1. Urachus
2. Right Ureter
4. Urethra
3. Left Ureter
The triangle defined by the connection of the two ureters
and the exit of the urethra is NOT ELASTIC. It is known
as the TRIGONE OF THE BLADDER.
The bladder is lined by a special type of epithelium:
TRANSITIONAL EPITHELIUM (it’s stretchy).
URETER ATTACHMENT
• Traverse the bladder obliquely.
• So, when bladder is full, they get squeezed
flat.
• There is no valve, but this passive closing
prevents urine from backing up into the
kidneys.
THE NEPHRON AND COUNTERCURRENT
EXCHANGE:
Countercurrent Exchange is one of the most important
physiological phenomena in all of nature. (Among other
things) It allows hyperconcentration of substances.
Definition:
COUNTERCURRENT EXCHANGE – a pair of adjacent
channels or tubes containing fluids flowing in opposite
directions and having an energetic gradient directed between
one channel and the other. (In other words, stuff can flow
between the tubes.)
Low concentration out
High concentration in
(fluid flow)
(fluid flow)
Low concentration in
High concentration out
The arrangement of tubes in OPPOSITE FLOW DIRECTIONS
means that even as the tube starts with lower concentration and picks
up solutes, even when it get to the end it can still pickup a bit, as it is
encountering the highest concentration of adjacent tube.
A COUNTERCURRENT EXCHANGE
SYSTEM can only work if there is an
asymmetry in the system.
In this case, there is active transport in only
one direction. In the case of the kidney, salts
are being transported to concentrate salts in
the urine.
Proximal Convoluted
Tubule:
Initiate concentration of
glomerular filtrate.
About 75% of sodium
removed by active
transport here, and
chlorine follows
passively.
Remaining fluid in
nephron tube is about
same concentration as
that of surrounding
interstitial fluid.
Remaining fluid reduce
to about 25% original
volume
Loop of Henle:
Acts in manner of counter
current exchanger. Note
that each limb of loop has
fluid moving in opposite
directions (even though
connected at one end).
Further concentrates
urine.
Also means that salt
concentration will be
highest near bend in the
loop.
DESCENDING LOOP OF HENLE: No active
transport of salt out of the descending loop of Henle.
ASCENDING LOOP OF HENLE: Chlorine ions
actively transported out of loop into the interstitial
space. Oppositely charged sodium ions follow.
However, water does not move out of the ascending
loop.
A concentration gradient IN THE INTERSTITIAL
SPACE has been set up by the chlorine ( plus
sodium) transport.
Distal Convoluted
Tubule:
In addition to sodiumchloride, potassium,
ammonia, and carbonate
removed from filtrate here.
These are retained as
needed by the body.
At this point, nephron has
used materials IN the
glomerular filtrate to set up
a concentration gradient in
the interstitial space of the
kidney.
Collecting Duct:
Receives many proximal
convoluted tubules.
Collecting duct now passes
through the concentration
gradient set up by many
adjacent nephrons.
So, as glomerular filtrate
passes down collecting
tubule, it moves through
higher and higher
concentration of sals that
were set up by loops of
Henle.
Direction of
fluid flow
through
By process of osmosis, water wants to move from region of higher
collecting water concentration to lower. This pulls water from filtrate, leaving
tubule.
behind a more concentrated “urine”.
By setting up a countercurrent system,
a salt concentration gradient is set up
in the interstitial space (highest
concentration near curve of loop of
Henle).
As fluid in collecting tubules reacts to
concentration gradient, water is pulled
out, concentrating filtrate as urine.
GROSS ANATOMY
DEFINE BY
MICROSCOPIC
STRUCTURE:
CORTEX – part that
contains the
convoluted tubules
and proximal part of
Loop of Henle, and
proximal part of
collecting tubules.
MEDULLA -- contains distal part of Loop of Henle, and distal part of
collecting tubules. Where most of the active transport takes place.
URINE PRODUCTION: HORMONAL CONTROL
In regards to urine production, the most important hormone is
ANTIDIURETIC HORMONE, or ADH.
ADH makes the collecting duct MORE permeable to water.
Thus, secretion of ADH causes the retention of water in the
body, and more concentrated urine. (ADH is usually secreted
in response to environmental situations that require the
retention of water.)
A diuretic will have opposite effect: decreases permeability
of collecting tubule, so you lose lots of water (copious, dilute
urine). Examples of diuretics: caffeine, alcohol (beer
particularly due to the hops), capsasin (the active ingredient
in hot peppers), others.